THE TROMBE WALL; A Good Idea Wherever It Travels

It's a long way from France, where the Trombe wall was conceived, to the Plateau of Ladakh in the Himalayas. But good ideas - simple, cost-effective, energy-saving ideas for the home - have a way of getting where they are most needed.

In recent years Felix Trombe's heat-collecting, heat-storage system has found its way to Ladakh, a territory southwest of Tibet governed by India. It is designed to take the bite out of the wind-swept winters of a high-altitude desert - and at a cost that is satisfying even in a third-world economy.

Professor Trombe, a key figure in France's solar movement and the prime mover behind the 20,000-mirror solar furnace at Font-Romeu Odeillo in the Pyrenees, designed his simple heat-gathering wall to be effective wherever the sun shines. It can be as ornate or as straightforward as taste and economic restrictions dictate.

The point is that it is simple enough to be applied to any building with a south-facing (or in the Southern Hemisphere, north-facing) wall, as its growing use in Ladakh illustrates.

In its simplest form the Trombe system is a wall of concrete or stone covered by double glazing.

The wall is generally painted black to improve its heat-gathering ability. The sun, passing through the glazing, strikes the wall, warming it throughout the day. Air between the glazing and the wall rises to pass into the room through vents by natural convection, drawing cooler air from the room through other vents placed near the floor.

At night the vents are closed off to prevent the convection system from reversing itself and cooling the house. But the heat stored in the stone, brick, mud wall, etc. (and this is the principal reason for the Trombe wall's success), radiates steadily into the house after the sun has gone down.

In almost treeless Ladakh, where temperatures often drop to 40 degrees F.below zero during the eight-month-long heating season, each house has traditionally kept one room warmed by a dung-fueled fire which is both smoky and inefficient. Because of the long heating season, dung fuel is also often in short supply, while imported coal is expensive.

In fact, ''whenever a Ladakhi expresses his most urgent needs, space heating is his top priority,'' according to Helena Norberg-Hodge, an American-born resident of England who introduced the solar-heating concept to the Ladakhis after she toured there with a German film crew and saw the obvious need.

In contrast, where Trombe walls have been installed the residents report pleasant temperatures, more evenly distributed throughout the house. They delight, too, in the certain knowlege that their fuel supply will never run out and that the atmosphere is smoke-free.

In 1979 the first two Trombes, built by consultants from the Intermediate Technology Development Group in London, were built in Leh, the Ladakh capital. The performance exceeded all expectations. In January 1980, when temperatures in a control building without heat had dropped to 12 degrees F., the coldest a Trombe-wall room dropped to was 40 degrees F. at the end of a long, cold night.

Average low temperatures in the Trombe homes were 45 degrees F., while average maximum daytime temperatures rose to 65 degrees F.

The relatively inexpensive cost of installing a Trombe wall (about $350) likewise appeals to the Ladakhis, who pay more than $200 a year on imported coal in those areas where dung supplies are not available. In other words, the cost of installing a Trombe wall is recovered in less than two heating seasons.

Ms. Norberg-Hodge, a linguist by profession, had only the most peripheral background in passive solar heat when she toured Ladakh as an interpreter with the German film crew. But the theory behind the Trombe wall is ''so straightforward, so simple,'' she says, ''that I could see how effective it would be in Ladakh - or anywhere, for that matter.''

Thus, she set the wheels in motion and began what she hopes will be a revolution in home heating in the high and arid plateau. The Ladakhis, she adds, ''could make better use of the dung in their farm soils.''

The point Ms. Norberg-Hodge made during a recent visit to the United States is that the Trombe-wall principle can be adapted to buildings in many parts of the world. Obviously, an existing stone wall can be turned into a Trombe at least cost. But one can be built where necessary, or, as is increasingly happening in all-wood houses, the heat storage is being provided by water-filled cans, steel drums, or fiber-glass tubes specially constructed for the purpose.

A point to remember: Water is heavy. Don't go stacking water-filled drums in your home without placing additional support beneath the floor.

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